Juggernaut,
Your are correct that Urethane is pretty tame, but charicteristic urethan reaction gives you grip earlier on the lane, and we did not want this. Combined with the strong asymmetric Insignia core, the early read with the urethane cover was too pronounced for the conditions the Desperado was aimed at, so we needed to alter the cover. We tested various methods of reducing the early read that urethane usually has, and adding silicon gave us the look we were after.
I'm not 100% sure, but this could be the first ball produced with silicon in the cover.
Cheers
Ryan